There is a growing interest in the application of imaging techniques to drug discovery and development to enable earlier decisions to be made on the suitability of a given drug for the treatment of disease in humans. PET has the ability to image pharmacological functions in vivo in conscious humans and is ideally suited to this task. Either the drug itself can be radiolabelled and its biodistribution in tissues measured directly, or alternatively, a given pharmacological target can be labelled, enabling quantification of exogenous drug interaction. A wide range of mathematical modelling techniques may be applied to the spatial and kinetic PET data giving rise to functional images or regional parameter values that reflect the pharmacology of the drug under study. Examples will be shown and some of the mathematical techniques behind quantification will be discussed.